In recent years, higher photographic speeds and more rapid development processing have become increasingly important with silver halide photographic materials, especially with the photographic materials used for making prints. In the past, photographic emulsions containing silver chlorobromides which are essentially silver iodide free have been used for making prints due to the increased rate of development which can be obtained with these materials. While many attempts have been made to also increase the photographic speed of such emulsions there have been other problems such as low contrast or pressure resistance.
For example, although the emulsions prepared by halogen conversion disclosed in JP-B-50-36978 have higher photographic speeds, it has been found that they are readily desensitized when pressure is applied to the photographic material. (The term "JP-B" as used herein signifies an "examined Japanese patent publication".)
Techniques involving so-called "laminated type emulsions", in which the grains have a layer of a different halogen composition over an interior silver halide grain, have been disclosed, for example, in JP-B-56-18939, JP-A-58-9137, JP-A-58-95736, JP-A-58-108533, JP-A-60-222844 and JP-A-60-222845. However, it has been found that the contrast is likely to be soft in the toe part of the characteristic curve. It has also been discovered that desensitization by pressure is likely to arise. (The term "JP-A" as used herein signifies an "unexamined published Japanese patent application".)
The halogen converted layer type silver chlorobromide emulsions disclosed in JP-A-63-282730 have been found to have excellent pressure related properties, but have been found to be inadequate in terms of providing suitable photographic speed and contrast.
The use of sulfur sensitization or selenium sensitization has been preferred for the chemical sensitization of silver iodobromide emulsions which are to be used for making prints, since fogging is less likely to occur when these methods are used. Furthermore, the presence during chemical sensitization of nitrogen containing heterocyclic compounds such as azaindene compounds (e.g., 4-hydroxy-6-methyl-1,3,3a,7-tetra-azaindene) and/or mercaptoazole compounds (e.g., 1-phenyl-5-mercaptotetrazole and 2-amino-5-mercapto-1,3,4-thiadiazole), has been desirable.
However, even when these techniques have been used conjointly, it has been very difficult to prepare silver halide emulsions wherein there is no loss of photographic speed or contrast, and in which fogging during development is suppressed as well.